However, OpenSSH certificates can be very useful for server authentication and can achieve similar benefits as the standard X. For user authentication, the lack of highly secure certificate authorities combined with the inability to audit who can access a server by inspecting the server makes us recommend against using OpenSSH certificates for user authentication. They also allow using strict host key checking, which means that the clients will outright refuse a connection if the host key has changed. Changing the keys is thus either best done using an SSH key management tool that also changes them on clients, or using certificates. Thus it is not advisable to train your users to blindly accept them. However, if host keys are changed, clients may warn about changed keys.Ĭhanged keys are also reported when someone tries to perform a man-in-the-middle attack. The host keys are almost always stored in the following files: They can be regenerated at any time. Each host can have one host key for each algorithm. Host keys are just ordinary SSH key pairs. Creating Host Keys The tool is also used for creating host authentication keys. A connection to the agent can also be forwarded when logging into a server, allowing SSH commands on the server to use the agent running on the user’s desktop.įor more information on using and configuring the SSH agent, see the ssh-agent page. Like this: During the login process, the client proves possession of the private key by digitally signing the key exchange.Īdding the Key to SSH Agent ssh-agent is a program that can hold a user’s private key, so that the private key passphrase only needs to be supplied once. The following commands illustrate: This can be conveniently done using the ssh-copy-id tool. The algorithm is selected using the -t option and key size using the -b option. Thus its use in general purpose applications may not yet be advisable. Support for it in clients is not yet universal. Most SSH clients now support this algorithm. Only three key sizes are supported: We would recommend always using it with bits, since the keys are still small and probably more secure than the smaller keys even though they should be safe as well. This is probably a good algorithm for current applications. DSA in its original form is no longer recommended. It is based on the difficulty of computing discrete logarithms.Ī key size of would normally be used with it. It is quite possible the RSA algorithm will become practically breakable in the foreseeable future. RSA is getting old and significant advances are being made in factoring.Ĭhoosing a different algorithm may be advisable. These include: A key size of at least bits is recommended for RSA bits is better.
Our online random password generator is one possible tool for generating strong passphrases. The passphrase should be cryptographically strong. The passphrase is used for encrypting the key, so that it cannot be used even if someone obtains the private key file. However, in enterprise environments, the location is often different. Here’s an example: Enter passphrase empty for no passphrase: Enter same passphrase again: The key fingerprint is: SSH keys for user authentication are usually stored in the user’s. In this case, it will prompt for the file in which to store keys. They should have a proper termination process so that keys are removed when no longer needed. Thus, they must be managed somewhat analogously to user names and passwords. However, SSH keys are authentication credentials just like passwords. It improved security by avoiding the need to have password stored in files, and eliminated the possibility of a compromised server stealing the user’s password. SSH introduced public key authentication as a more secure alternative to the older. The authentication keys, called SSH keys, are created using the keygen program. Such key pairs are used for automating logins, single sign-on, and for authenticating hosts. Command and Option Summary What Is ssh-keygen? Ssh-keygen is a tool for creating new authentication key pairs for SSH.